Re: Bypassing cursors in postgres_fdw to enable parallel plans

At 2026-07-03 20:51:12，"Rafia Sabih" <rafia(dot)pghackers(at)gmail(dot)com> wrote ：
>
> No, but I don't understand why you think they can be different in the
> current implementation.
> Currently, as per init_scan fsstate->conn_state->active_scan = fsstate, and
> it is never reassigned or anything.
> So they cannot have different conn_state.
> Am I missing something?
>

This is the reproduction procedure for the crash caused by clearing active_scan that I mentioned last week.

Data preparation:
CREATE TABLE local_big (id int, val text);
CREATE TABLE local_big2 (id int, val text);
INSERT INTO local_big SELECT i, 'val_' || i FROM generate_series(1, 300000) i;
INSERT INTO local_big2 SELECT i, 'val_' || i FROM generate_series(1, 300000) i;
CREATE SERVER test_srv FOREIGN DATA WRAPPER postgres_fdw
OPTIONS (host 'localhost', port '55436', dbname 'test_bug2');
CREATE USER MAPPING FOR CURRENT_USER SERVER test_srv;
CREATE FOREIGN TABLE ft_big (id int, val text)
SERVER test_srv
OPTIONS (schema_name 'public', table_name 'local_big',
streaming_fetch 'true', fetch_size '1');
CREATE FOREIGN TABLE ft_small (id int, val text)
SERVER test_srv
OPTIONS (schema_name 'public', table_name 'local_big2',
streaming_fetch 'true', fetch_size '1');

session-1

DROP TABLE IF EXISTS bug2_pid;
CREATE TEMP TABLE bug2_pid (pid int);
INSERT INTO bug2_pid VALUES (pg_backend_pid()); -- pid_1

DO $$
DECLARE cnt int;
BEGIN
SET LOCAL enable_hashjoin = off;
SET LOCAL enable_mergejoin = off;

SELECT count(*) INTO cnt
FROM ft_big b, ft_small s
WHERE b.id = s.id
AND pg_backend_pid() > 0;

RAISE NOTICE 'completed: cnt=%', cnt;

EXCEPTION WHEN OTHERS THEN
RAISE NOTICE 'caught: %', SQLERRM;
END;
$$;

SELECT 'post_check' AS phase, count(*) FROM ft_big; -- CRASH HERE

session-2
SELECT pid FROM bug2_pid;
SELECT pg_cancel_backend(pid); -- After Session 1 enters the drain loop, execute pg_cancel_backend(pid) in Session 2.

I believe the issue regarding fsstate->conn and active_fsstate->conn that you've been discussing with Robert is related to this crash.
If only one PgFdwScanState* (active_fsstate) is available inside save_to_tuplestore, the assignment active_scan = NULL will clearly not be executed prior to draining.
The draining logic operates on data from active_fsstate, and we can only clear it after draining completes successfully.

Best regards,

--

Yilin Zhang